1. Field of the Invention
This invention relates to enhancing the performance of a Global Positioning System (GPS) receiver module, and more particularly to enhancing reception of the GPS signals or using other information to determine the location of a GPS receiver module.
2. Background of the Invention
Global Positioning System (GPS) is a multiple-satellite based radio positioning system in which each GPS satellite transmits signals that allow locations to be determined by measuring the distance from selected GPS satellites. The satellites are positioned in a constellation such that typically seven, but a minimum of four satellites will be observable by a user anywhere on or near the earth's surface. Therefore, the GPS satellites provide GPS signals that may be received globally on a continuous basis. The GPS signals are carried over two frequencies known as L1 (1575.42 MHz) and L2 (1227.6 MHz) using spread spectrum techniques that employ spreading functions.
GPS is intended to be used in a wide variety of civilian and military applications such as vehicle navigation, precise positioning, time transfer, attitude reference, and surveying. FIG. 1 is a diagram illustrates an application of tracking parcels 122 transported via a vehicle 120. GPS receiver modules are contained or secured to the parcels 122 to determine and track the location of the parcels 122 based on the GPS signals from a constellation of satellites 110.
A GPS receiver module comprises a number of components including, among other components, an antenna assembly, an RF assembly, and a GPS processor. The antenna assembly receives the L-band GPS signal and feeds it to the RF assembly. The RF assembly mixes the L-band GPS signal down to a signal of intermediate frequency (IF). Using various known techniques, the PRN code modulating the L-band signal is tracked through code-correlation to measure the time of transmission of the signals from the satellite. The GPS processor differences the measured time of transmission with the time of reception and determines a pseudo range between the receiver and the satellite. This pseudo range includes both the range to the satellite and the offset of the receiver's clock from the GPS master time reference. The GPS processor computes a three dimensional position based on the pseudo range measurements and navigation data from multiple satellites.
Reception of the GPS signals at the GPS receiver module is vital to determining accurate location of the GPS receiver. For various reasons, however, the quality of GPS signals received at the GPS receiver module may be degraded. One of the causes of poor reception is the non-optimal orientation of the antenna. If the GPS antenna is not oriented toward the satellites, the reception of the GPS signals may be poor. FIG. 2 illustrates a graph illustrating an antenna pattern of a conventional GPS antenna. The GPS antenna of FIG. 2 has good signal reception up to approximately 100 degrees from the pattern maximum (at 0 degree) but degrades significantly above approximately 100 degrees and reaches the pattern minimum at 180 degrees from the pattern maximum. In this range between approximately 100 degrees and 180 degrees, the signal reception is degraded significantly. If the GPS signals are received in this poor reception range, the reception of the GPS signals degrades significantly. Although the reception may be improved by manually reorienting the GPS antennas so that the poor reception range does not face the satellites, the manual adjustment of the GPS antennas may not be practicable or possible in some applications.